A steel pipe for a line pipe used for a pipeline that transports crude oil and natural gas for a long distance is required to achieve an increase in strength or thickness in terms of transport efficiency. For increasing the strength of steel, it is effective to utilize bainite, and high strength welded steel pipes with X80 of the American Petroleum Institute (API) specification and higher have been developed.
In the case when increasing the thickness of the steel pipe for a line pipe, a decrease in toughness of a weld portion, particularly, a weld heat affected zone (Heat Affected Zone (“HAZ” hereinafter)) becomes a problem. In the case when a thick plate is formed into a pipe shape by a UO process, and edge portions of the thick plate are jutted to each other to be seam welded, submerged arc welding with a large amount of heat input is employed. In the above submerged arc welding, toughness decreases due to coarsening of a structure of the HAZ and formation of a mixed structure of martensite and austenite (Martensite-Austenite constituent (“MA” hereinafter)) in particular.
Particularly, in the case when seam welding the thick plate by submerged arc welding from an inner surface and outer surface of the plate, the HAZ, which is welded first, in the proximity of a weld fusion line is reheated by heat input of the welding to be performed later. As above, in the case when the submerged arc welding is performed a plurality of times, there is sometimes a case that the coarse structure of the HAZ becomes an origin of fracture, and the toughness decreases markedly.
Further, in a high strength welded steel pipe with X80 or higher, bainite is utilized for increasing its strength, and thus a decrease in toughness of a base material and a HAZ becomes a problem. With respect to such a problem, for improving the toughness of the base material, there has been proposed a method of utilizing polygonal ferrite. Further, for improving the toughness of the HAZ, there has been proposed a method of utilizing intragranular transformation to make a structure fine (for example, Patent Documents 1 to 4). Further, there has been proposed a method of improving toughness of a HAZ by addition of B (for example, Patent Document 5).